Localised optimal initial perturbations are studied to gain an understanding of the global stability properties of streamwise corner-flow. A self-similar and a modified base-flow are considered. The latter mimics a characteristic deviation from the self-similar solution, commonly observed in experiment. Poweriterations in terms of subsequent direct and adjoint linearised Navier-Stokes solution sweeps are employed to converge optimal solutions for two optimisation times. The optimal response manifests as a wave packet that initially gains energy through the Orr mechanism and continues growing exponentially thereafter. The study at hand represents the first global stability analysis of streamwise corner-flow and confirms key observations made in theoretical and/or experimental work on the subject. Namely, the presence of an inviscid instability mechanism in the near-corner region and a destabilising effect of the characteristic mean-flow deformation found in experiment. / <p>QC 20130604</p>

La récupération assistée du pétrole implique la progression dans unmilieu poreux d'une phase aqueuse qui pousse une phase organique. Al'échelle du pore, les forces visqueuses et la gravité sontnégligeables, et la capillarité joue un rôle prépondérant : ledéplacement des fluides est gouverné par leur affinité avec lessurfaces et par les hétérogénéités de canaux. Les films liquidesexistent dans les pores d'une roche. Trois systèmes microfluidiquessont mis en place pour étudier le rôle des films microscopiques (demouillage) ou des films macroscopiques (de coins). Le premier consisteen un capillaire de section circulaire dans lequel on suit lemouvement d'un ménisque dans des conditions de mouillage variées. Enparticulier, en mouillage pseudo-partiel, un hystérésis d'angle decontact est observé, mais les films de mouillage présents dans cesystème conduisent à un accrochage de ligne de contact beaucoup plusfaible qu'en absence de film. Les deuxième et troisième systèmesmettent en évidence l’influence des films de coins dans un canalrectangulaire sur l'avancée du ménisque. Le couplage de l'écoulemententre la phase du coin et la phase du milieu entraîne le drainage del’huile piégée. Les propriétés de ce nouveau mécanisme sontcaractérisées par des expériences et s’accordent avec le modèleconstruit. / Enhanced oil recovery involves the displacement of an organic fluid byan aqueous one in the pores of the rocks. At the pores scale. Thedisplacement of fluids is governed by wetting condition of the system.The viscous and gravity forces are negligible and the capillarityplays a dominant role heterogeneities of channel sizes. Threemicrofluidic systems are designed to study experimentally the role ofmicroscopic liquid films (wetting) or macroscopic ones (corners) inthe pores level. The first one consists of the displacement of ameniscus in a circular capillary with various wetting conditions. Inparticular, for pseudo-partial wetting systems, a contact anglehysteresis is observed but with a weak pinning as compared to partialwetting systems where there are non wetting films. The second andthird ones show the influences of liquid films in the corners of asquare channel. The coupling between the corner flows and the mainflow involves the drainage of the trapped oil cluster. The propertiesof this new mechanism are consistent with the theoretical model, andalso characterized by experiments.

Evaporation is a key process for the water exchange between soil and atmosphere, it is controlled by the internal water fluxes and surface vapor fluxes. The focus of this thesis is to visualize and quantify the multiphase flow processes during evaporation from porous media. The retained liquid films in surface roughness (thick-film flow) and angular corners (corner flow) have been found to facilitate and dominate evaporation. Using the representative 2D micromodels (artificial pore networks) with different surface roughness and pore structures, this thesis gives visualizations of the corner and thick-film flow during the evaporation process, presents the enhanced hydraulic continuity by corner and thick-film flow, and tests the validity of the SSC-model which assumes corner flow is dominant for the mass transport during evaporation. Surface roughness and wettability are proved both experimentally and theoretically to play a key role for the time and temperature behaviors of the evaporation process, besides, this thesis shows that for a consistent description of the time-dependent mass loss and the geometry of the corner/thick-film flow region, the fractality of the evaporation front must be taken into account.